Phase modulation system for combining carrier wave segments containing selected phase transitions

ABSTRACT

A phase modulation system is described in which the phase of a carrier wave can assume one of a number of permitted values in each symbol period and wherein transitions between the permitted values occur at transition periods between symbol periods. Difficulties associated with phase transitions are reduced or overcome by storing or generating the transitions with a number of cycles of the carrier wave of the appropriate phase(s) on either side thereof, a wave train being built up by merging the corresponding oscillations of the same phase which succeed and preceed respectively two adjacent transitions.

This application is a continuation-in-part of application Ser. No.809,365 filed June 23, 1977 and now abandoned.

The present invention relates to the production of a phase modulatedsignal.

When information is transmitted in the form of phase modulation of acarrier wave, a requirement exists for fast switching from one phase toanother at transitions between adjacent phase values, or "symbols".Typically the symbol periods are short and a difficulty arises ingenerating acceptable waveforms at the transitions.

It is an object of this invention to overcome or reduce the difficultyreferred to above.

According to the invention from one aspect there is provided signallingapparatus for producing a wave train which maintains one of a pluralityof predetermined phase positions representing respective symbols duringsymbol transmitting periods, and which changes from one to another ofsaid phase positions only during short transition periods between saidsymbol transmitting periods, said apparatus comprising waveform sourcemeans which makes separately available a plurality of differentoscillations, respectively defining all the possible phase transitions,each oscillation including a respective phase transition and a pluralityof cycles at both sides of that transition the phases at the respectivesides defining that transition, and selecting means for changing fromone of said oscillations to another during symbol transmitting periods,the two oscillations between which changing occurs being in the samephase position while changing occurs and the second oscillationundergoing the desired phase transition during the next transitionperiod.

According to the invention from another aspect there is provided a datatransmission arrangement for transmitting a carrier wave, phasemodulated by the data, such that, in each of a plurality of adjacentsymbol periods, the phase of said carrier wave measured relative to areference carrier wave having the same frequency of oscillation assumesone of a plurality of permitted values representing respective symbolswith phase transitions occurring between the symbol periods, including asource of separate waveforms defining respective ones of said phasetransitions, each waveform including a respective phase transition andat both sides of the transition, a plurality of cycles of oscillation ofthe carrier wave frequency phases at the respective sides defining saidtransition, means for selecting respective ones of said waveforms, andmeans for combining the waveforms appropriate to successive transitionsby combining the cycles of oscillation of these separate waveforms whichrelate to the same symbol.

An embodiment of the invention is applicable to systems in whichinformation is transmitted in the form of oscillations (at the carrierwave frequency) with one of four phases (measured relative to referenceoscillations) differing by 90° during each of a number of successivesymbol periods. If the symbol periods are very short, (for exampleextending over only a few cycles of the carrier wave), the switchingbetween successive phases for different symbols must be very fast andthis can give rise to difficulties in achieving acceptable waveformsover the transition between phases. Since, in this system, the phase inone symbol period can assume any of the four permitted values, as canthe phase in the succeeding symbol period, there are sixteen possiblecombinations of first and second states which have to be allowed for,from one symbol to the next.

The embodiment overcomes the difficulty of fast phase switching byhaving available stored replicas of the waveforms required for thetransitions between different phases. These waveforms can be designed tohave whatever characteristics are considered desirable for thetransition between phases, and include more than one half of each of thesuccessive symbols separated by the transition. The required signal isgenerated by merging successive stored replicas over the interval whenthey are nominally of the same phase (i.e. over the overlapping parts ofthe same symbol). In this way any switching is done relatively slowlyand is only between nominally inphase states.

In order that the invention may be clearly understood and readilycarried into effect, some embodiments thereof will now be described byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 represents, in simplified and schematic form, part of a phasemodulated carrier wave,

FIG. 2 shows a store containing waveforms which include possible changesfrom a first to a second symbol phase,

FIG. 3 shows how waveforms, derived from the store of FIG. 2 in theappropriate order, can be combined to synthesize the modulated waveshown in FIG. 1,

FIG. 4a is a symbolic representation of a surface acoustic wave (SAW)device for use in the invention, FIG. 4b is a representation of awaveform producible in the SAW device of FIG. 4a, and

FIG. 5 is a block diagram of a practical embodiment of the inventionusing SAW devices of the type shown in FIG. 4.

Referring now to FIG. 1, the various rectangles shown therein are all ofequal length and represent a sequence of successive symbol periods inaccordance with the prescribed rate of data transmission. The capitalletter A, B, C or D in each rectangle represents one of four permittedvalues of the phase of the carrier wave. It will thus be seen that thephase in the successive symbol periods shown are A,B,C,C,B,D and A;representing part of a sequence extending over a considerably longertime than represented by the seven time intervals shown.

Referring now to FIG. 2, there is represented a waveform source means inthe form of a matrix 1 of sixteen surface acoustic wave (SAW) devices 11for producing sixteen waveforms, each waveform representing a transitionfrom one of the permitted phases to another e.g. AB or a maintaining ofthe same phase e.g. DD as the case may be. The SAW devices 11 arerepresented as grouped in rows and columns for simplicity although itwill be appreciated that such grouping is not intended to infer thatthey are actually arranged in such a format or even that they arearranged in a two-dimensional matrix at all.

An address selector 2 is supplied with information concerning thedesired phases in each two successive symbol periods and generates anaddress signal which calls up the waveform producible at the desiredlocation of the matrix and applies that waveform to a wave trainsynthesizing circuit 3. Each waveform produced by the SAW devices, infact comprises not only a transition between two permitted phases, butalso contains a number of cycles of the carrier wave frequency, with theappropriate phase, at respective sides of the transition. Each SAWdevice is arranged to produce a waveform lasting for between one and ahalf to two symbol periods (i.e. the transition and betweenthree-quarters and one symbol period either side thereof). The phases ofthe waveforms produced by the SAW devices must be fixed, relative to areference oscillation of the same frequency, and thus it is desirablethat there is an integral number of cycles of the carrier wave frequencyin each symbol period.

The wave train synthesizing circuit 3 assembles the waveforms producedsuccessively by the SAW devices 11 to construct the wave train shown inFIG. 1 by adding the oscillation following one transition with theoscillation preceding the next transition; both oscillations are atleast nominally of the same phase because they represent the permittedphase value prevailing in a common symbol period, and because of theaforementioned arrangement that there is an integral number of cycles ofthe carrier wave in each symbol period.

If the wave train is being synthesized at a relay station to replicatedata transmitted from a remote station, the symbol periods are definedby the incoming data. Since the waveforms have been generated locally,the frequency of the carrier wave produced by assembling the waveformsmay not correspond accurately with an integral multiple of the symbolperiods defined by the incoming carrier wave. In these circumstances thetwo waveforms to be added in synthesizing the content of the train ofFIG. 1 during two successive symbol periods may not be exactly in phasedue to timing errors. To avoid phase discontinuity problems, the twooscillations should have suitable amplitude weightings applied beforeadding, the first oscillation amplitude being caused to graduallydecrease whilst the second oscillation gradually increases, theamplitudes being substantially complementary. This is symbolicallyrepresented in FIG. 3 by the sloping lines, on the waveforms produced bythe matrix, in the regions of overlap of those waveforms. The slopes arelinear in the drawing but this need not be the case, and (for example)cos² θ and sin² θ waveforms can be used if desired.

An examplary embodiment of the invention will now be described withreference to FIGS. 4 and 5.

The apparatus of FIG. 5 comprises a matrix 51 of 16 SAW devices 511which are connected to an impulse generator 52 to selectively receiveelectrical impulses. The devices 511 are arranged to produce amplitudeweighted waveforms including phase transitions (as described above) inresponse to the impulses.

FIG. 4a is a symbolic representation of such an SAW device arranged toproduce in response to an impulse an amplitude-weighted waveformincluding a phase transition as shown in FIG. 4b. The manner ofconstruction of such an SAW device is known to those skilled in the artof SAW devices. For instance, an article in "IEEE Transactions onMicrowave Theory and Techniques, Vol.MTT-21, No.4, April 1973" on pages162 to 175, entitled "Impulse Model Design of Acoustic Surface-WaveFilters" by Hartmann et al, discusses an SAW device for producing anamplitude weighted waveform in response to an impulse. The same articlerefers to phase weighted SAW devices. Furthermore, another article onpages 263 to 271 of the same issue of those IEEE Transactions entitled"Application of Acoustic Surface-wave Technology to Spread SpectrumCommunications" by Bell et al, discloses an SAW device for producing abiphase phase-shift-keyed signal waveform comprising 100 symbol periods(or chips as they are called in that article).

In essence, each device comprises a piezoelectric substrate 41 on asurface of which two inter-digital transducers are deposited. Onetransducer 42 is a launch transducer having a broad frequency responseand is arranged to receive an electrical impulse applied across its pads421 and 422 by the impulse generator 52. The inter-digital fingers ofthe transducer 42 convert the electrical impulse into an acoustic wavewhich propagates in the surface region of the substrate 41. The othertransducer 43 is arranged to convert the acoustic wave into a desiredelectrical signal having an amplitude-weighted waveform including aphase transition. The output of the transducer 43 comprises its pads 431and 432.

Referring again to FIG. 5, the matrix 51 comprises SAW devices 511 forproducing respective amplitude-weighted waveforms including the 16possible phase transitions between the permitted symbols A to D. Thesewaveforms are produced in response to impulses selectively received bythem from the impulse generator and are applied to respective ones of 16buffer amplifiers 58. The outputs of the amplifiers 58 are connected tothe waveform synthesising circuit in tne form of an analogue adder 3.Switches 53, which are illustrated as mechanical switches, but inpractice would be, in this example, electronic switches of known form,select which SAW device is to receive an impulse at any one time. Theswitches 53 are controlled by the address selector 2.

The address selector 2 comprises a phase sensitive detector 54 having aninput 541 for receiving the phase modulated waveform (as shown in FIG.1, for example) to be relayed, and a reference input 542 for receiving aphase reference signal. The detector 54 produces an analogue outputrepresenting the symbols A to D. An analogue to digital converter 55converts the output of the detector to digital form. As only the foursymbols A to D are involved in this example, the converter outputs a2-bit code representing each symbol A,B,C or D. A succession of codes isproduced in response to the waveform received at the input 541. In orderto identify each transition between the phases, it is necessary to storethe pairs of symbols giving rise to that transition. For this purposethe codes are serially fed to a 4-bit shift register 56. In this way 16different four bit codes representing the respective different possibletransitions between the four symbols A to D are produced. These four bitcodes are fed to an address converter 57. The converter 57 has sixteenoutputs connected to the respective switches 53. It produces in responseto any one of the 16 different four bit codes representing a particularphase transition a control signal on that output connected to the switch53 which is connected to the SAW device 511 arranged to produce thewaveform including that transition. The switch 53 connects itsassociated SAW device 511 to the generator 52 in response to the controlsignal.

Various modifications of the preceding embodiment and alternativeembodiments will be apparent to those skilled in the art.

For instance, instead of selectively impulsing the 16 SAW devices 511,the devices could be simultaneously impulsed and the desired outputsselected by a suitable commutator connected between the SAW devices andthe adder 3.

Instead of incorporating the amplitude weighting in the design of theSAW devices, means may be provided externally of the devices toamplitude weight their outputs, the devices having a linear amplituderesponse.

If the same symbol were to prevail over three or more successive symbolperiods, then the same SAW device is required to respond to a secondimpulse whilst still providing an output signal in response to theprevious impulse. If the SAW device has completely linear amplituderesponse this does not cause any problems, but in practice it can bemore acceptable to provide two separate SAW devices for such aneventuality. Since the sequence of symbol phases is known in advance,any switching between the SAW devices can easily be controlled by thedata stream itself. Only the lines providing the phase transitions A/A,B/B, C/C and D/D would have to be duplicated.

The number of SAW devices required can be reduced by making them doubleended, i.e. placing receiver transducers at either end and a launchertransducer in the centre of the substrate of each device. Theaforementioned impulses can then be applied in the central transducerand respective phase transition waveforms can be obtained from the pairsof transducers at the ends of the substrate. By this means, it ispossible to reduce to ten the number of SAW devices nominally required(i.e. not allowing for the possibility of duplication in the case ofsuccessive transitions such as A/A). If A,B,C and D represent quadraturephases taken in order (e.g. 45°,-45°,-135°135°, respectively) this isachieved by providing one device disposed to give A/A and B/Btransitions from its respective end transducers, another similar deviceto give C/C and D/D transitions from its end transducers, four devicesgiving respectively A/B, B/C, C/D and D/A transitions (the sametransitions occur at both end transducers for these devices two devicesgiving respectively A/C, D/B and B/D, C/A transitions at their endtransducers and finally two devices giving respectively A/D, C/B andB/A, D/C transitions at their end transducers. The number of SAW devicesnominally necessary can be further reduced to six, by suitable use ofphase inverter circuits connected to selected output transducers of thedevices. Selection of the required transitions is effected, in this caseat the output end of the devices, all devices being impulsedsynchronously.

In order to allow for the fact that some of the SAW devices will berequired to receive a drive impulse before they have finished supplyingoutput signals in response to a preceding pulse, it is necessary toplace a duplicate set of receiving or output transducers one symbolperiod "downstream" of each original output transducer. The output istaken from one of these duplicate transducers and a further impulse isnot applied to the device. When inverters are used, therefore, a totalof six substrates are required on which there are twelve dedicatedinverters. The number of inverters could be reduced by multiplying apair if preferred. The period over which errors can build up phasedifferences is, however, increased.

If the phase differences due to variations in the symbol rate or carrierwave frequency become significant, it can be necessary to introduce thefollowing additional feature, in which each output transducerarrangement is replaced symmetrically by a pair of such arrangementswhich are separated (temporally if not physically) by, say, λ/12=±15°.The pairs are selected in exactly the same way as was the singletransducer arrangement they replace, except that the two outputs aretransferred separately to the appropriate one of two pairs of variableweighting circuits. The function of each of these pairs of phaseweighting circuits is to differentially weight the two inputs so thatwhen they are subsequently added, the resultant is of any desired delayor "phase" between them. The impressed delay is then swept linearly withtime, passing through zero at the instant corresponding to the symboltransition. The magnitudes of the slopes appropriate to the original and"downstream" transducer arrangement are in a fixed proportion(determined by the geometry) and are driven by a sampling loop detectingthe residual phase difference during the changeover period. A SAW devicecan be used here to differentially delay one channel by λ/4 with respectto the other for driving a quadrature phase detector. Frequencyvariations still lead to phase errors but only by virtue of a λ/4 pathdifference rather than a 7λ or 19λ path difference, so that the residualphase error is reduced by about 30 to 80 times.

If the four permitted phases are ±45°,±135°, they can be synthesised byadding together one vector drawn from 0° or 180° and another, of equalamplitude, from ±90°.

To generate the transitions corresponding to the ±90° vector it isnecessary to have one line containing essentially two output transducerarrangements to generate 90°/90° and -90°/-90° phase transitions andanother line comprising one output transducer arrangement to generate a90°/-90° phase transition from which an inverter will produce theremaining transition -90°/90°. Since the input electrode arrangementsare pulsed on only alternate symbol transitions, it is necessary toduplicate the output electrode arrangements downstream. The 0° or 180°transitions are produced in a similar way. This technique leads to aneven simpler arrangement than the system described previously.

As an alternative to adding externally, it is possible to arrange forthe sixteen pairs of vectors to be added together while still in SAWform. In this case, the pairs of SAW waveforms could either be launchedin parallel to intercept double-length output electrode arrangements orcould be directed in opposite directions down the same path so as tomeet at the output arrangement.

What I claim is:
 1. Signalling apparatus for producing a wave trainwhich maintains one of a plurality of predetermined phase positionsrepresenting respective symbols during symbol transmitting periods, andwhich changes from one to another of said phase positions only duringshort transition periods between said symbol transmitting periods, saidapparatus comprising waveform source means which makes separatelyavailable a plurality of different oscillations respectively definingall the possible phase transitions, each oscillation including arespective phase transition and a plurality of cycles at both sides ofthat transition the phases at the respective sides defining thattransition, and selecting means for changing from one of saidoscillations to another during symbol transmitting periods, the twooscillations between which changing occurs being in the same phaseposition while changing occurs and the second oscillation undergoing thedesired phase transition during the next transition period.
 2. A datatransmission arrangement for transmitting a carrier wave, phasemodulated by the data, such that in each of a plurality of adjacentsymbol periods the phase of said carrier wave measured relative to areference carrier wave having the same frequency of oscillation assumesone of a plurality of permitted values representing respective symbolswith phase transitions occurring between the symbol periods, including asource of separate waveforms defining respective ones of said phasetransitions, each waveform including a respective phase transition andat both sides of the transition a plurality of cycles of oscillation ofthe carrier wave frequency the phases at the respective sides definingsaid transition, means for selecting respective ones of said waveforms,and means for combining the waveforms appropriate to successivetransitions by combining the cycles of oscillation of these separatewaveforms which relate to the same symbol.
 3. An arrangement accordingto claim 2 wherein said source of waveforms comprises at least onesurface acoustic wave device having input and output transducer meansand wherein the input transducer means receives electrical impulsesignals.
 4. An arrangement according to claim 3 wherein the surfaceacoustic wave device is arranged to produce an amplitude-weightedwaveform including a phase transition.
 5. An arrangement according toclaim 3 wherein the input transducer means is a launch transducer havinga broad frequency response and the output transducer means is a receivertransducer defining the waveform which said device is arranged toproduce.
 6. An arrangement according to claim 3 wherein said input andoutput transducer means are located adjacent opposite ends of saidsurface acoustic wave device.